If BCH is so great, why is the price (against BTC or USD) so low?

Although there are a lot of reasons to expect BCH to outcompete BTC as the real Bitcoin and become global reserve currency in the long run, it is still true that so far BCH has performed poorly on the open market, particularly with regards to the exchange rate.

Even if it's true that BCH is not dead, BTC has not won & BCH can flip BTC, doesn't this market vote empirically demonstrate that BCH isn't that great after all?

How can the price be so low if BCH supporters are correct that it's much better than BTC?

There are a number of factors to consider:

• Initial snowball: The BTC community, through the use of censorship & backroom dealing, came out on top in the initial civil war. They received the existing Bitcoin brand, exchange ticker (and more importantly, all of the trade liquidity & technical integrations), control of the Github repository, majority of the mining hash rate, control of central (at the time) discussion platforms /r/Bitcoin & bitcointalk. The network effect of both communication platforms & monetary liquidity is individually an enormous hurdle to overcome - let alone both at once - and BCH thus understandably struggled at first which has compounded over time.
• Community chaos & division: ~.
• Catch-up delays: Due to being the minority coin, BCH's early days as an independent community involved a lot of time and effort in shoring up the initial deficit. Time was spent on establishing a governance process, coalescing on an altered brand, technical upgrades such as new address formats & improving the difficulty adjustment algorithm. These are not small changes, much less when requiring coordination across a decentralised (and at the time internally divided) ecosystem. Time, money and effort expended in these areas was obviously at the opportunity cost of upgrades to maintain competitive in the fast moving cryptocurrency market. The silver lining of these changes is that it was a one-time cost & paying this price came with some small benefits (BCH difficulty adjustment is smoother than BTCs for example so it handles hashrate fluctuations better).
• Unrealised future benefit: The initial reason to split, increasing the blocksize, was ironically hardly beneficial - then or since. The majority of the 1MB / block transaction traffic stayed on BTC, . The BCH community is playing for keeps, shooting for global reserve currency over a period of decades, and its initial reason to break away provided no immediate benefit. In the same way that an expert chess player might sacrifice a valuable piece in the short term to gain a long term positional advantage, BCH had to suffer in the short term to maintain its long term scaling viability. Much as with the chess players the benefit of this kind of move is only apparent to a sophisticated observer thinking over the long run (which the market does not). Consider the popular saying "In the short term, the market is a popularity contest & in the long term a weighing machine". Note that the situation is reversed for BTC. Their stagnant technical choices and social consensus were beneficial at the time of the split - was reinforced by the short-term momentum - but are now becoming deep sticking points as their scaling plans falter & they are intractably bogged down in their past decisions.
• Ongoing sabotage attempts: This item is more debatable, but a conspiratorial reading of events suggests that censorship & community disruption of Bitcoin is a targeted attack by government or banking operatives. As BTC was no longer a legitimate threat to state power once its scaling capacity was limited, their community presumably no longer had to navigate around these subvertive adversaries. BCH meanwhile likely continued (and continues) to be maliciously meddled with, which possibly explains (at least in part) the community's further splits & difficulties in coordinating. It's far easier to make progress without state interference, and where BTC likely had none since the split, BCH has likely had significant attention. Individuals can judge for themselves to what extent, if any, this was a factor - but those who believe it was can take heart that it confirms BCH remains as disruptive as the pre-split Bitcoin.

Consider also that BCH is nowhere near as far behind as a simple price chart would suggest.

• Conceptual similarity: Most of the fundamental concepts of Bitcoin are shared between the coins. A 21 million limit, proof of work mining, decentralised community, self custody of funds, how to use a wallet, the problems with fiat currency and so on. All the education & branding & trust built by the much-larger BTC community incidentally benefits the credibility of BCH to almost the same extent. Once someone is convinced Bitcoin is a viable idea and/or knows how to use it - the jump from BTC to BCH is a trivial adjustment. This is one of the critical reasons the BCH community remains defiant with its branding.
• Shared mining: All of the innovation, investment & work that goes into the BTC mining industry (funded by BTC speculators and users paying high transfer fees) is received by BCH completely for free. As and when BCH price begins to rise, miners can trivially switch their entire operation to BCH in a matter of hours to days by simply downloading a BCH node to use instead of their BTC node.